[SPRING, 2014] ASSIGNMENT
PROGRAM – BCA (REVISED FALL 2012)
SEMESTER - II
SUBJECT CODE & NAME - BCA2010 – OPERATING SYSTEM
CREDIT – 4 BK ID - B1405 MAX. MARKS =60
Q. No 1. Define operating system. Explain any four major functions of operating system
2+8 = 10
Answer: Operating System is a System Software (Set of system programs) which provides an
environment to help the user to execute the programs. An operating system (OS) is a software program
that manages the hardware and software resources of a computer. A key component of system software,
the OS performs basic tasks, such as controlling and allocating memory, prioritizing the processing of
instructions, controlling input and output devices, facilitating networking, and managing files.
Functions of operating system:
Processor Management
Memory Management
Input and Output Management
File System Management
Establishment and Enforcement of a Priority System
The functions of Operating systems are explained below:
i)
Processor Management: It means assigning processor to different tasks which has to be
performed by the computer system.
ii)
Memory Management: It means allocation of main memory and secondary storage areas
to the system programmes, as well as user programmes and data.
iii)
Input and Output Management: It means co-ordination and assignment of the different
output and input devices while one or more programmes are being executed.
iv) File System Management: Operating system is also responsible for maintenance of a file
system, in which the users are allowed to create, delete and move files.
v)
Establishment and Enforcement of a Priority System: It means the operating system
determines and maintains the order in which jobs are to be executed in the computer system.
2 What is hardware virtual machine? Discuss the three ways through which the software
virtualization can be done 4+6=10
Answer:
Hardware Virtual Machine
The original meaning of virtual machine, sometimes called a hardware virtual machine, is that of a
number of discrete identical execution environments on a single computer, each of which runs an
operating system (OS). This can allow applications written for one OS to be executed on a machine which
runs a different OS, or provide execution "sandboxes" which provide a greater level of isolation between
processes than is achieved when running multiple processes on the same instance of an OS. One use is to
provide multiple users the illusion of having an entire computer, one that is their "private" machine,
isolated from other users, all on a single physical machine. Another advantage is that booting and
restarting a virtual machine can be much faster than with a physical machine, since it may be possible to
skip tasks such as hardware initialization.
Software virtualization can be done in three major ways:
ation, or "full virtualization with dynamic recompilation"– the virtual
machine simulates the complete hardware, allowing an unmodified OS for a completely different CPU to
be run.
-virtualization – the virtual machine does not simulate hardware but instead offers a special API
that requires OS modifications. An example of this is XenSource's XenEnterprise (www.xensource.com)
ve virtualization and "full virtualization"– the virtual machine only partially simulates enough
hardware to allow an unmodified OS to be run in isolation, but the guest OS must be designed for the
same type of CPU. The term native virtualization is also sometimes used to designate that hardware
assistance through Virtualization Technology is used.
3 Explain the following scheduling algorithm [10 Marks].
a) First come First served
b) Shortest Job First 5+5=10
Answer:
a) First-Come-First-Served Scheduling Algorithm
This is one of the brute force algorithms. A process that requests for the CPU first is allocated the CPU
first. Hence, the name first come first serve. The FCFS algorithm is implemented by using a first-in-firstout (FIFO) queue structure for the ready queue. This queue has a head and a tail. When a process joins
the ready queue its PCB is linked to the tail of the FIFO queue. When the CPU is idle, the process at the
head of the FIFO queue is allocated the CPU and deleted from the queue. Even though the algorithm is
simple, the average waiting is often quite long and varies substantially if the CPU burst times vary greatly.
Shortest-Job-First Scheduling
Another approach to CPU scheduling is the shortest job first algorithm. In this algorithm, the length of the
CPU burst is considered. When the CPU is available, it is assigned to the process that has the smallest next
CPU burst. Hence this is named shortest job first. In case there is a tie, FCFS scheduling is used to break
the tie.
The SJF algorithm produces the most optimal scheduling scheme. For a given set of processes, the
algorithm gives the minimum average waiting and turnaround times. This is because, shorter processes
are scheduled earlier than longer ones and hence waiting time for shorter processes decreases more than
it increases the waiting time of long processes.
The main disadvantage with the SJF algorithm lies in knowing the length of the next CPU burst. In case of
long-term or job scheduling in a batch system, the time required to complete a job as given by the user can
be used to schedule. SJF algorithm is therefore applicable in long-term scheduling. The algorithm cannot
be implemented for CPU scheduling as there is no way to accurately know in advance the length of the
next CPU burst. Only an approximation of the length can be used to implement the algorithm. But the SJF
scheduling algorithm is provably optimal and thus serves as a benchmark to compare other CPU
scheduling algorithms. SJF algorithm could be either preemptive or non-preemptive. If a new process
joins the ready queue with a shorter next CPU burst then what is remaining of the current executing
process, then the CPU is allocated to the new process. In case of non-preemptive scheduling, the current
executing process is not preempted and the new process gets the next chance, it being the process with the
shortest next CPU burst.
4. Define deadlock. Discuss any four conditions that lead to deadlock 4+6= 10
Answer: Definition:
Several processes compete for a finite set of resources in a multi-programmed environment. A process
requests for resources that may not be readily available at the time of the request. In such a case the
process goes into a wait state. It may so happen that this process may never change state because the
requested resources are held by other processes which themselves are waiting for additional resources
and hence in a wait state. This situation is called a deadlock.
Necessary Conditions for Deadlock
A deadlock occurs in a system if the following four conditions hold simultaneously:
1) Mutual exclusion: At least one of the resources is non-sharable, that is, only one process at a time can
use the resource.
2) Hold and wait: A process exists that is holding on to at least one resource and waiting for an additional
resource held by another process.
3) No preemption: Resources cannot be preempted, that is, a resource is released only by the process that
is holding it.
4) Circular wait: There exist a set of processes P0, P1… Pn of waiting processes such that P0 is waiting for
a resource held by P1, P1 is waiting for a resource held by P2… Pn-1 is waiting for a resource held Pn and
Pn is in turn waiting for a resource held by P0.
5 Explain LRU page replacement algorithms with an example 4+6= 10
Answer:
LRU page replacement algorithm
The main distinction between FIFO and optimal algorithm is that the FIFO algorithm uses the time when
a page was brought into memory (looks back) whereas the optimal algorithm uses the time when a page is
to be used in future (looks ahead). If the recent past is used as an approximation of the near future, then
replace the page that has not been used for the longest period of time. This is the least recently used
(LRU) algorithm.
Illustration:
Reference string: 7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1
Memory frames: 3
Page faults: 7 7 7 2 2 4 4 4 0 1 1 1
0000 0 0033 300
11 3
3222
227
Number of page faults = 12.
The LRU page replacement algorithm with 12 page faults is better than the FIFO algorithm with 15 faults.
The problem is to implement the LRU algorithm. An order for the frames by time of last use is required.
Two options are feasible:
In the first option using counters, each page table entry is associated with a variable to store the time
when the page was used. When a reference to the page is made, the contents of the clock are copied to the
variable in the page table for that page. Every page now has the time of last reference to it.
According to the LRU page replacement algorithm the least recently used page is the page with the
smallest value in the variable associated with the clock. Overheads here include a search for the LRU page
and an update of the variable to hold clock contents each time a memory reference is made. In the second
option a stack is used to keep track of the page numbers. A page referenced is always put on top of the
stack. Therefore the top of the stack is the most recently used page and the bottom of the stack is the LRU
page. Since stack contents in between need to be changed, the stack is best implemented using a doubly
linked list. Update is a bit expensive because of the number of pointers to be changed, but there is no
necessity to search for a LRU page.
LRU page replacement algorithm does not suffer from Belady’s anomaly. But both of the above
implementations require hardware support since either the clock variable or the stack must be updated
for every memory reference.
6 What is computer virus? List the types of virus and its various infection methods. 4+6 =
10
Answer: Computer Virus
A computer virus is written with an intention of infecting other programs. It is a part of a program that
piggybacks on to a valid program. It differs from the worm in the following ways:
ly whereas virus does not operate
independently.
as well as data.
Types of viruses
There are several types of computer viruses. New types get added every now and then. Some of the
common varieties are:
Infection methods
Viruses infect other programs in the following ways:
Append: virus code appends itself to a valid unaffected program
Replace: virus code replaces the original executable program either completely or partially
Insert: virus code gets inserted into the body of the executable code to carry out some undesirable
actions
Delete: Virus code deletes some part of the executable program
Redirect: The normal flow of a program is changed to execute a virus code that could exist as an
appended portion of an otherwise normal program.